The present invention relates to a platinum thin film resistance element for use in a temperature sensor and a gas sensor and a method for the manufacture of such a platinum thin film resistance element.
As a resistance thermometer for use in temperature sensors, there has heretofore been employed a platinum winding resistance element or negative temperature coefficient temperature sensitive resistance element (for example, a thermistor). These elements, however, have the following defects:
Main defects of platinum winding resistance element:
(1) Since its impedance is usually as small as 50 or 100.OMEGA., the output sensitivity is low.
(2) It is susceptible to mechanical vibration or shock; namely, since a platinum wire is wound on a glass rod, the platinum wire is likely to slip off from the glass rod due to vibrations.
(3) Since the resistance value is small, a three-core or four-core lead wire is needed for avoiding the influence of a lead wire.
Main defects of thermistor:
(1) Compatibility is poor because of difficulty in the production of thermistors of the same characteristics.
(2) Characteristic variations with time are substantial.
(3) The temperature-resistance characteristic is negative and exponential rather than linear.
Gas sensors employing such conventional resistance elements have the following defects:
A sensor using a platinum wire coil is called a hot-wire gas sensor. A catalyst is laid on the platinum wire coil and, upon arrival of a gas, the catalyst promotes its combustion to cause a change in the resistance value of the platinum winding, and this resistance variation is detected.
(1) Because of the winding, its resistance value cannot be increased. Therefore, a voltage of a bridge circuit for detecting the resistance variation is as low as 2 V or so on an average and the gas sensitivity is also poor; it is impossible to detect a low concentration of a gas the molecular heat of combustion of which is low, such as carbon monoxide.
(2) Since it is necessary that the platinum wire be wound into uniform coils, with their catalyst coated surfaces held in the same condition, the productivity is poor and the mechanical strength is low.
A sensor using a thermistor is called a gas thermal conductivity system. This makes use of a difference in thermal conductivity between air and a gas to be sensed and requires two thermistors of the same characteristics.
(1) It is difficult to select two thermistors of the same characteritics.
(2) The balance between the two thermistors is lost owing to characteristic variations with time.
(3) The output is not linearly proportional to the gas concentration.
A sensor using a metal oxide is called a semiconductor sensor and is intended to directly read out a resistance variation which is caused by the adsorption of a gas to a metal oxide such as SnO.sub.2, ZnO, V.sub.2 O.sub.5 or the like. This sensor also has the following drawbacks:
(1) Much time is required until it becomes stable after the connection of the power source.
(2) The zero point is very unstable even in the absence of the gas to be sensed.
(3) The low-concentration sensitivity is good but an output change with a concentration change at a high concentration is very small.
(4) Reproducibility is very poor and hence reliability is low.
Heretofore, there has not been put to practical use a gas sensor which is capable of accurately detecting carbon monoxide even at such a low concentration as 50 PPM without being affected by other gases.
Similarly, there has not been available a highly stable and sensitive sensor for detecting a nitrogen oxide, in particular, nitrogen monoxide. Further, there have not been proposed a sensor capable of stably detecting only ammonia even at a low concentration or a sensor capable of stably detecting only an inflammable gas.
A conventional platinum resistance element has employed a winding resistor and it has been said that a thin film resistance element could not be produced. That is, even if a platinum thin film is deposited by sputtering on an insulating substrate as is the case with the fabrication of the conventional thin film resistance element, the platinum thin film is not held stably and disappears during heat aging.
It is an object of the present invention to provide a platinum thin film resistance element the resistance value of which can easily be made large and which does not require a three-core or four-core lead wire but is stable.
Another object of the present invention is to provide a method for the manufacture of a stable platinum thin film resistance element.
Another object of the present invention is to provide a stable and reliable platinum thin film resistance element capable of accurately detecting a gas to be detected.
Another object of the present invention is to provide a platinum thin film resistance element which is capable of accurately detecting carbon monoxide of low concentration.
Another object of the present invention is to provide a platinum thin film resistance which is capable of stably detecting a nitrogen oxide.
Another object of the present invention is to provide a platinum thin film resistance element which is capable of detecting an ammonia gas even at a low concentration.
Yet another object of the present invention is to provide a platinum thin film resistance element which is capable of stably detecting an inflammable gas alone.